78 G Sir Oliver Lodge on the Transmission 



question of a possible opacity or obstruction to the passage 

 of waves due to this cause, we must realize that solar radia- 

 tion consists not of light alone but probably of a, /3, and 

 7 rays also. Ultra-violet light, and a rays if they reach us, 

 will be active ionizers in the upper regions of air, but the 

 more penetrating kinds of radiation should ionize much 

 more than a superficial layer, and when the beam of sun- 

 shine is horizontal a great deal of atmosphere may be 

 affected. 



By some such extra-terrestrial influence, in addition to 

 any effect of emanations from the earth's crust, the ioniza- 

 tion of the atmosphere may be preserved and constantly 

 renewed, in spite of the clearing-away tendency of Hertzian 

 waves. And if these waves thus have a continually renewed 

 set of charged particles to deal with, they will be virtually 

 encountering a constantly increasing mass ; and some amount 

 of their momentum and energy must be absorbed by trans- 

 ference to the particles. 



Now we know, from Maxwell, Poynting, and others, that 

 the energy of radiation per unit volume may be regarded as 

 a momentum carried across unit area per second ; in other 

 words, as a pressure or potential pressure in the direction of 

 travel. This pressure P may be expressed either as an 

 energy density, i. e. in ergs per c.c; or as a pressure, in 

 dynes per square centimetre ; or as a surface density mul- 

 tiplied by an acceleration; or as a volume density multiplied 

 by the square of a velocity ; or merely as the momentum 

 travelling per second through unit area. If no obstacle is 

 encountered, the radiation momentum is merely passed on 

 in the ether undiminished, and the potential pressure is not 

 mechanically exerted ; but if matter is present, some of it is 

 transferred to matter, and the etherial power will to that 

 extent diminish, even without any production of heat. 



It has long been known that small particles are most 

 readily moved by the pressure of light, but now we see that 

 charged ions of low atomic weight, and especially electrons, 

 are more affected than anything else; especially by long 

 waves. 



The momentum transmitted per second to matter will be 

 a certain fraction of Py, and will equal the material momentum 

 generated in the same time, Nmviv ; where mw is the 

 momentum attained by each particle, and Nu the number 

 freshly encountered per unit area per second. 



We may express it better thus : — 



The loss of P per centimetre is dT/dx, and this will be 

 accounted for by the gain of momentum generated per 



